Sensorineural hearing loss (SNHL) affects 30 million people in the United States, and over 275 million people worldwide, leading to a significantly lowered quality of life and a large economic burden. For those with severe to profound SNHL, a cochlear implant is currently the only available route for restoring `adequate' functionality. Only 310,000 implants had been done worldwide till 2012, and about 50,000 devices are being sold annually. This large discrepancy between those that can benefit and those that actually get the implant is driven partly by their cost, availability and the lack of trained surgeons and facilities. However, there are critical limitations in the current devices which contribute significantly to very low adoption: (a) The external microphone does not localize sound from the auricle and ear canal, introducing unnatural cues; (b) The cumbersome, highly visible unit attached to the skull outside the mastoid bone, connected by a cable to the sound processor and rechargeable battery placed behind the ears, makes the disability very obvious, and creates an unacceptable social stigma for many patients, causing them to refuse this option; (c) The device must be taken off during showering or swimming (although a few water-resistant versions are now available), and during sleeping, increasing the likelihood of damage, misplacement, or the ability to communicate in these situations. (d) Having an external magnet can lead to skin breakdown and implant extrusion (e) The large footprint and placement leaves open the risk for device migration requiring revision surgery. A fully-internal cochlear implant (FICI), would address this large unmet medical need. The lack of a self-contained power supply is a key roadblock preventing realization of all FICIs. Based upon our extensive experience with fabricating lithium ion battery (LIB) electrodes, and our synthesis of several novel solid polymer electrolytes specifically targeted for LIBs, we hypothesize that an all-solid LIB with a LiFePO4 cathode, a Li5Ti4O12 anode and a lithium salt doped (poly) ethylene oxide solid electrolyte, will meet all of the electrochemical, safety and footprint requirements for this application. At Audiance, Inc. we have the engineering, electrochemical and medical expertize, as well as the facilities to fabricate this battery, characterize its performance and iterate on processing strategies and component loadings until it meets or exceeds all of the metrics required for a FICI. The all-solid LIB will remove one of the most critical roadblocks for the development of a FICI, and thus open up an option for many patients around the world with severe to profound SNHL. The battery design strategy could also be used for other implants that have similar stringent performance metrics.